Gallium nitride (GaN) and aluminum gallium nitride (AlGaN) high electron mobility transistors (HEMTs), silicon carbide (SiC) and other high voltage transistors are becoming popular for high voltage power conversion applications, due to high breakdown voltages as well as low on state resistance and reduced conduction losses. Electron trapping in AlGaN/GaN HEMTs causes current collapse and increased drain-source on-state resistance (RDSON) in certain dynamic conditions. However, measuring the dynamic RDSON performance of HEMTs is difficult. Measurements by semiconductor testers do not simulate the true device conditions in actual power electronics circuits. For example, switching transistors in typical power converters undergo a hard-switching transition before turning on. The transistors also switch at high frequencies, typically hundreds of KHz, and thereby the measurement needs to reflect the RDSON value within a very short time after turn on, such as a microsecond in some instances. These conditions are very difficult to replicate in semiconductor testers, particularly for testing multiple devices. Improved circuits and techniques for measurement of the dynamic RDSON under the operating conditions of real power electronics circuits are therefore desired, particularly for HEMTs such as high-voltage AlGaN/GaN and SiC transistors. One approach is to measure the on-state drain-source voltage and the corresponding transistor current during dynamic operation. However, the drain voltage in high voltage applications varies between hundreds of volts in the off state and millivolts in the on state. As a result, direct measurement using ordinary oscilloscope voltage probes can saturate the oscilloscope channel when the high-voltage transistor is off. Moreover, the measured drain-source transistor voltage cannot be accurately measured by conventional high voltage oscilloscope probes due to their high divide ratios, typically 100×, resulting in signals too small for the oscilloscope to resolve when the transistor is on. Conventional voltage clamping circuitry can be used to limit the maximum voltage seen by the oscilloscope, but these circuits introduce large RC time constants and thus do not provide sufficiently short settling times to accurately assess dynamic drain-source voltage characteristics and hence dynamic RDSON for high-voltage AlGaN/GaN and SiC transistors in real-world conditions.